This proposal seeks to define the molecular basis of expression of the leukocyte integrin molecule CD11a (LFA- 1, leukocyte function antigen- 1) during human monocyte/macrophage development. CD11a is the alpha subunit of the CD11a/CD18 heterodimer that is expressed on the cell surface of both lymphocytes and monocyte/macrophages. The CD11a/CD18 complex mediates homotypic and heterotypic leukocyte adherence by binding to one of two defined ligands, intercellular adherence molecule (ICAM) 1 or 2, on a conjugate cell. CD11a has been shown to be involved in a variety of adherence-related leukocyte activities involved in host defense, including monocyte to endothelial adhesion, microbe and tumor cell killing by macrophages, and cytotoxic reactions by T-lymphocytes. Expression of the CD11a subunit in monocyte/macrophage cells is developmentally regulated. Immature myeloid precursor cells express very low levels of CD11a, whereas terminally differentiated macrophages, such as the alveolar macrophage, express high levels of the CD11a subunit. Despite the importance of the CD11a molecule in monocyte/macrophage- adherence related functions, the regulatory sequences and factors responsible for the expression of the CD11a subunit during macrophage development have not been identified. The studies described in this proposal are designed to identify and characterize the regulatory sequences, and ultimately the factors, responsible for expression of the CD11a molecule during human monocyte/macrophage development. Initial studies will focus on regulation at the level of transcription initiation, with the specific aims of: 1) identifying the CD11a transcription start site, 2) identifying the CD11a promoter and enhancer regions using transient expression assays, and 3) identifying specific regions of DNA-protein interactions in the CD11a promoter/enhancer using DNAase I footprinting and gel shift analysis. The role of the CD11a molecule in normal macrophage-mediated host defenses indicates that understanding this developmental process may have clinical importance, in addition to providing insight into a fundamental biologic process.